src: enable clear memory on all platforms

- convert `_libssh2_explicit_zero()` to macro. This allows inlining
  where supported (e.g. `SecureZeroMemory()`).

- replace `SecureZeroMemory()` (in `wincng.c`) and
  `LIBSSH2_CLEAR_MEMORY`-guarded `memset()` (in `os400qc3.c`) with
  `_libssh2_explicit_zero()` macro.

- delete `LIBSSH2_CLEAR_MEMORY` guards, which enables secure-zeroing
  universally.

- add `LIBSSH2_NO_CLEAR_MEMORY` option to disable secure-zeroing.

- while here, delete double/triple inclusion of `misc.h`.
  `libssh2_priv.h` included it already.

Closes #810

src/mbedtls.c

@@ -96,12 +96,8 @@ _libssh2_mbedtls_safe_free(void *buf, int len)
     if(!buf)
         return;
 
-#ifdef LIBSSH2_CLEAR_MEMORY
     if(len > 0)
         _libssh2_explicit_zero(buf, len);
-#else
-    (void)len;
-#endif
 
     mbedtls_free(buf);
 }